Electronic circuits contain many (sometimes millions) of components such as resistors, capacitors, inductors, diodes, electro-mechanical switches, and transistors. High density packaging of electronic components is particularly important to allow fast access to large amounts of data in computers. High density electronic circuit packages also are important in high frequency devices and communications devices. The components are connected to form circuits and circuits are connected to form functioning devices. The connections perform power and signal distribution. In a multi-layer electronic circuit package, some layers of the package serve as conductors and other layers serve as signal planes, depending on the operational requirements of the device. The devices require mechanical support and structural protection. The circuits themselves require electrical energy to function. The functioning devices, however, produce heat, or thermal energy which must be dissipated so that the devices do not stop functioning. Moreover, while high density packaging of a number of components can improve performance of the device, the heat produced by the power-consuming components can be such that performance and reliability of the devices is adversely impacted. The adverse impact arises from electrical problems such as increased resistivity and mechanical problems such as thermal stress caused by increased heat.
Electronic circuit packages, such as chips, modules, circuit cards, circuit boards, and combinations of these, thus must meet a number of requirements for optimum performance. The package must be structurally sturdy enough to support and protect the components and the wiring. In addition, the package must be capable of dissipating heat and must have a coefficient of thermal expansion that is compatible with that of the components. Finally, to be commercially useful, the package should be inexpensive to produce and easy to manufacture.
High density packages necessarily involve increased wiring density and thinner dielectric coatings between layers in a multi-layer electronic circuit package. The layers in a multi-layer package typically are electrically connected by vias and through-holes. The term "via" is used for a conductive pathway between adjacent layers in a multi-layer electronic circuit package. The term "through-hole" is used for a conductive pathway that extends to a non-adjacent layer.
Presently, a widely used technique for achieving such electrical connection, is to provide vias having a metal plated on the walls of the vias. However, plating is a relatively expensive process including the processing steps of cleaning, seeding or catalyzing the walls and then plating with the desired metal such as copper. Because of the relative expense associated with plating, alternatives have been suggested to provide electrical conductivity in unplated vias, whether through-holes or blind vias. The alternative methods include providing solder-paste-filled blind vias, solder balls in blind vias, solder-filled through-holes, solder rings, and copper powder mixed into solder paste. Each of these methods depend on the solder bridging unplated/unsoldered prepreg.
For high density packages the through-holes are increasingly narrow in diameter and the through-holes in each layer must be aligned precisely. The through-holes and vias are likely to be plated at least where they connect at either end. The routing of lines in a dense electronic circuit package is made more difficult by surface joining features that typically are formed as lands connected to plated through-holes or vias.
The land and through hole/via features also limit the density of the interconnect pattern. More particularly, each conductor site must be separated from other sites by a certain amount of dielectric material which limits the density of the sites.